Producing a copolyester from two lower dialkyl esters of dicarboxylic acid and a glycol

ABSTRACT

A process for producing a copolyester by copolymerizing the monomers from the ester interchange of at least two lower dialkyl esters of dicarboxylic acid and glycol using a novel catalyst system. The novel catalyst composition includes from about 20 ppm to about 150 ppm manganese; from about 50 ppm to about 350 ppm lithium; preferably from about 10 ppm to about 70 ppm cobalt; and from about 200 ppm to about 400 ppm antimony, all amounts being based upon the expected yield of the copolyester. The novel process to make copolyester includes the step of employing an effective catalytic amount of manganese and lithium in an ester interchange reaction where the lower dialkyl esters and glycol produce a monomer mixture; and using an effective catalytic amount antimony in a copolycondensation reaction of the monomers.

BACKGROUND OF THE INVENTION

This patent is a continuation-in-part of U.S. Pat. application Ser. No.07/489,583 filed Mar. 7, 1990 and issued Feb. 5, 1991 as U.S. Pat. No.4,990,594.

FIELD OF THE INVENTION

The present invention relates to a process for producing a copolyesterfrom two lower dialkyl esters of a dicarboxylic acid (LDE) and a glycol(GLY). Using a specific catalyst system of the present invention allowsto conveniently produce the copolyester by reacting the two lowerdialkyl esters of dicarboxylic acid with glycol in an ester interchangereaction. In particular, the catalyst system comprises manganese (Mn),lithium (Li), antimony (Sb) and optionally cobalt (Co). Morespecifically, manganese and lithium are used as catalysts for the esterinterchange of the LDE and GLY, while lithium, cobalt and antimony areused as catalysts for the polycondensation stage. The manganese issequestered after the ester interchange.

Furthermore, copolyesters of the present invention are demonstrated tohave improved melt flow characteristics and in solid form, improvedcutting characteristics during processing and increased tensile strengthwhen bonded in a blend with other polyester fibers.

Copolyesters of the present invention can be used as binder fibers innonwoven applications or can be used in conjunction with polyester fiberin bicomponent fibers.

PRIOR ART

In prior art processes, copolyesters can be produced by two differentroutes: ester interchange plus polycondensation, referenced herein asthe ester interchange route; or direct esterification pluspolycondensation, referenced herein as the direct esterification route.Generally, the ester interchange route can be used with either a batchtype process or a continuous process, while the direct esterificationroute uses a continuous type process.

In the ester interchange route, dimethyl terephthalate (DMT), dimethylisophthalate (DMI) and ethylene glycol (EG) are typically reacted in thepresence of a catalyst (manganese) at atmospheric pressure and at atemperature of from about 180° C. to 230° C. In the presence of thecatalyst, these components undergo ester interchange to yield twointermediate monomers and methanol. The reaction which is convenientlydone with about 0.6 mole of DMT, about 0.4 mole of DMI and 2.8 to 3.2moles of EG, is reversible and is carried to completion by removingmethanol formed. During the ester interchange, the two intermediatemonomers are the substantial majority product (not considering themethanol), along with small amounts of low molecular weight oligomers.

The monomers are then polymerized by a polycondensation reaction, wherethe temperature is raised to about 270° C. to about 300° C. and thepressure is reduced to below 1 mm of mercury vacuum and in the presenceof a suitable polymerization catalyst (antimony). From this reaction,polyethylene terephthalate/isophthalate) and ethylene glycol are formed.Because the reaction is reversible, the glycol is removed as it isevolved, thus forcing the reaction toward the formation of thepolyester.

Manganese is the preferred catalyst for ester interchange reactions, butthe amount of manganese employed must be strictly controlled. Thepresence of too little manganese during the ester interchange reactionresults in very long reaction times, while the presence of too muchmanganese results in unwanted side products during the polycondensationreaction, and unacceptable degradation of the copolyester resulting inpoor color (thus lowering the quality of the copolyester). The exactrange of manganese which proves to be the most desirable must generallybe determined through trial and error because many factors affect thereactivity of the manganese. For example, reaction temperature, reactionpressure, the degree of mixing during reaction, the purity of the rawmaterials, the presence of other additives, etc., all affect theeffectiveness of manganese.

In prior art process, manganese was employed to obtain suitable esterinterchange reaction times, but the manganese must be sequestered afterester interchange or during polycondensation by a polyvalent phosphorouscompound to aid in reducing the discoloration and unwanted sideproducts. Generally, prior art processes employed about 50 ppm to 150ppm manganese based on he expected yield of the polymer, as the esterinterchange catalyst. Using more than about 150 ppm manganese resultedin polymer degradation even if phosphorous was employed in excess tosequester the manganese. It is believed that this occurred because thephosphorous was incapable of complexing with the manganese to the degreenecessary to prevent discoloration.

Another method to make copolyesters is by direct esterification. By thismethod, terephthalic acid (TA), isophthalic acid (IPA) and EG aretypically reacted without any catalyst in a continuous process.Normally, the TA and IPA are reacted at a pressure of from about 5 psiato 85 psia and at a temperature from about 185° C. to 290° C. Thesecomponents undergo direct esterification to yield two intermediatemonomers and water. The reaction is conventionally done with amounts ofTA and IPA corresponding to the desired mixture in the copolyester. Forexample, if a 60/40 (terephthalate/isophthalate) is desired, then 1.2mole of TA, .8 mole of IPA and from 2.4 to 3.2 moles of EG are reacted.

After the completion of the direct esterification, the monomers are thenpolymerized by the polycondensation reaction as described in the esterinterchange process.

One use of copolyesters is in thermally bonded fibrous nonwovenapplications such as medical face masks wherein polyester fibers arethermally bonded to copolyester binder fibers that have been blendedwith the polyester fibers. The thermal bonding is attributed to thecopolyester fiber having a lower melting point than the blendedpolyester fiber. An example of such a copolyester is poly(ethyleneterephthalate/isophthalate) having a terephthalate/isophthalate moleratio from about 80/20 to 50/50.

Nonwoven products may also be formed of bicomponent fibers having apolyester core and a copolyester sheath. Such bicomponent fibers act asthe binder fibers when blended with polyester fibers to make nonwovens.

The following references are directed to various DMT type processes andcatalyst systems used for either making polyester or copolyester.

U.S. Pat. No. 3,709,859 to Hrach et al discloses a multi-componentcatalyst system for producing polyester via the ester interchangeprocess. Among the many catalysts mentioned are lithium, cobalt,manganese and antimony. Although these catalysts are set forth in thebackground portion of the patent, the patent claims a catalyst systemcomprising antimony, lead, and calcium, and additionally strontium orbarium. Hrach et al also teach the necessity of employing pentavalentphosphorous compounds as stabilizers in order to prevent the formationof discolored polyester.

British Patent 1,417,738 to Barkey et al discloses a process formanufacturing polyester in which a preferred ester interchange catalystsmay include zinc, manganese, cobalt, and lithium, among others.Preferred polycondensation catalysts include antimony compounds. Thisreference, however, claims other catalyst compounds and mentions theabove catalyst only as background information.

Various patents assigned to Eastman Kodak Company (British Patents1,417,738, and 1,522,656; U.S. Pat. Nos. 3,907,754, 3,962,189, and4,010,145) disclose a broad variety of catalyst systems, including amanganese, cobalt, lithium and titanium combination and a manganese,titanium, cobalt and antimony catalyst system, with phosphorous beingused in each of these systems as a sequestering agent. Each of thesecatalysts was added at the beginning of ester interchange and are usedas catalysts for the ester interchange. The purpose of the catalystsystem is to speed up the production of the copolyester.

The following references teach preparation of a polyester by the esterinterchange process with the addition of small amounts of TA in thepolycondensation step utilizing various catalyst systems.

U.S. Pat. No. 3,657,180 to Cohn discloses a process for makingcopolyester resin in which lithium or a divalent metal compound areemployed as catalyst. The specification states that manganese may be oneof the divalent metallic compounds which can be employed. The order ofmixing the various raw materials and the addition of the compounds tothe process described in the Cohn invention is stated to be critical.The process is carried out by reacting DMT and ethylene glycol in thepresence of a lithium salt under ester interchange conditions followedby the addition of manganese. In another embodiment, the process alsoincludes using manganese as a catalyst with lithium being added afterthe ester interchange reaction. In either case, the second metal isalways added after ester interchange, and thus is not used as a catalystfor the ester interchange. Moreover, the second metal is added in ahigher than catalytic amount and is added to act as a slip agent. Thesecond metal is added along with a slurry in the amount of less than 1%of product of glycol and a small amount of terephthalic acid beforevacuum-let-down to provide slip for polyester film and the amount addedis several factors larger than catalytic amount.

Although the catalyst systems disclosed in the prior art describedherein may be suitable for reducing ester interchange time, they arefound to differ from the catalyst system used in the present invention.As will be shown in the Examples, it has been found that the timing ofthe addition of the polycondensation catalysts also contribute to theproduction of the copolyester.

It is an aim or aspect of the present invention to not only feasiblyproduce a copolyester by the LDE type process from available suitableraw materials, but produce a copolyester which has acceptable color, IVand thermal properties.

SUMMARY OF THE INVENTION

The present invention provides a unique process of preparing acopolyester by effectively reacting two dialkyl esters of dicarboxylicacid and glycol by the ester interchange followed by thepolycondensation reaction through the use of a unique catalyst system.In particular, the present invention comprises a process for makingcopolyester wherein a catalyst consisting of manganese and lithium areused for the ester interchange reactions and the catalysts of cobalt(optional) and antimony are employed in the polycondensation reaction.

In the broadest sense, the present invention comprises a method formaking a copolyester from at least two lower dialkyl esters ofdicarboxylic acid (LDE) and a suitable glycol comprising the steps of:reacting the glycol with the LDE at a molar ratio of between about 1.8/1to about 2.5/1, at a suitable temperature and pressure and in thepresence of an effective amount of manganese and lithium catalystssufficient to produce a monomer mixture and alcohol; removing theresultant alcohol to more completely react the LDEs and glycol; reducingthe pressure to a vacuum pressure sufficient to initiatepolycondensation; copolymerizing the resultant monomer mixture at asuitable temperature and pressure and in the presence of an effectiveamount of antimony catalyst and optionally, but preferably a cobaltcatalyst to form a copolyester.

The present invention also comprises a copolyester product made by theabove-mentioned process.

The present invention also comprises a bicomponent fiber having apolyester core, a copolyester sheath made by the above-mentionedprocess.

Such copolyester and bicomponent fibers can be blended with staplecrimped polyester fibers and thermally bonded into nonwoven fabrics.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A copolyester of the present invention is prepared from at least twolower dialkyl esters of dicarboxylic acid and glycol. Dicarboxylic acidssuitable include acids having the formula ##STR1## wherein R is aselected from the group consisting of ##STR2## and --(CH₂)_(n) wheren=2-12 These dicarboxylic acids include terephthalic acid, isophthalicacid, adipic acid and naphthalic acid.

Suitable LDE's include dimethyl terephthalate, dimethyl isophthalate,diethyl terephthalate, dipropyl terephthalate, dibutyl terephthalate,dialkyl naphthalates such as 2.6 dimethyl naphthalate, or mixtures oftwo or more of these.

The glycol (GLY) may comprise ethylene glycol, diethylene glycol,polyethylene glycol, blends of ethylene glycol and propane and/or butanediol, or mixtures of two or more of these.

The catalyst system used in the present invention comprises from about20 ppm to about 150 ppm manganese (Mn); from about 50 ppm to about 350ppm lithium (Li); optionally, but preferably from about 10 ppm to about70 ppm cobalt (Co); and from about 200 ppm to about 400 ppm antimony(Sb), based upon the expected yield of the copolyester. This catalystsystem, when used in the most effective amounts, increased the esterinterchange rate, and polymerization rate, thereby reducing time toperform these functions.

Generally, the Mn and Li are added before the beginning of or during theester interchange reactions. At the end of es4 ter interchange or anytime before polycondensation, the manganese is sequestered in thosesituations where polymer color is important by the addition of asequestering agent, discussed in more detail later. Also, the manganeseis sequestered to allow for optimum rates for polycondensation. The Liremains in the reaction and acts as a catalyst in the polymerization.The Sb can be added before the beginning of or during polymerization asexplained more fully later and is a catalyst only for thepolycondensation. The Co can be added at any time subsequent to thestabilization of the manganese. In the past Co has generally beenconsidered to be an ester interchange catalyst. In the presentinvention, in the presence of Li and Mn, Co is found to delay the esterinterchange reaction but is effective as a polycondensation catalyst.

When stating that the manganese and lithium can be added at any timebefore or during ester interchange, it is intended to include theaddition of the catalyst metals in the LDE, glycol, or other feedstockmaterial streams. For example, a part, or all the ester interchangecatalysts could be added into the glycol feedstream. Moreover, if thefeed stream would also include other additives such as colorants,delustrants, opaquing agents, etc., the catalyst for the esterinterchange process (manganese and lithium) could be a part of theadditive feed stream.

When stating that cobalt and antimony can be added at any time before orduring polymerization, it is intended to include the fact that antimonycan be added at any time, including with the other metal catalyst ofmanganese and lithium, in the LDE, glycol, or other ester interchangefeedstock material streams. Cobalt, on the other hand, must be addedonly after substantial completion of the ester interchange reaction. Theaddition of cobalt before substantial completion of ester interchangeretards the ester interchange reaction rate. Thus, whether antimony isadded with other catalysts in the feestock streams, or is added with thecobalt after the ester interchange reaction does not make any differencein the copolyester produced.

Although metals are described for the catalyst system of the presentinvention, the catalyst may be added in the form of many differentcompounds. For example, compounds such as oxides and acetates are themost preferred, while such organic and inorganic compounds ascarbonates, phosphates (except manganese phosphates), halides, sulfides,amines, compounds of Group VI, etc., may also be employed. Preferably,manganese, lithium, and cobalt are added as catalysts in the form ofacetates, while antimony is generally added in the form of antimonyoxide. All catalyst compounds can also be used in the glycolized form ofpre-reacting with glycol. When a catalyst is added in the form of acompound, the amount of compound added is determined by the amount ofmetal catalyst desired and the amount of metal catalyst available in thecompound.

Other additives may be included in the general procedure outlined above,such as coloring agents, delustrants, opaquing agents, stabilizers, etc.These additives do not add or detract from the present invention.

In the process of the present invention, GLY and two LDEs are reacted ina batch process at temperature of between 150° C. to 250° C. atapproximately atmospheric pressure in an ester interchange reaction toproduce a monomer mixture and alcohol. The LDE and GLY are reacted inthe presence of manganese and lithium and are generally reacted in amolar ratio of GLY to LDE total, for example, of about 1.8/1 to 2.5/1.Because the ester interchange reaction is reversible, it is necessary toremove the alcohol formed to assure that the reaction favors theformation of monomer bis (2-hydroxyethyl) terephthalate (when usingdimethyl terephthalate and ethylene glycol).

It is theorized that lithium initiates the ester interchange reactionbetween the LDE and the glycol at a lower temperature than the effectivetemperature range of manganese. Although Applicants do not wish to bebound by this theory, it is believed that the addition of lithium andmanganese in the ester interchange reaction increases the esterinterchange rate thereby reducing the ester interchange time.

The reactivity of manganese as a catalyst occurs at a higher temperaturethan that of lithium. Manganese has a very high reactivity in both theester interchange reaction and the polycondensation reaction. It isgenerally preferred to sequester the manganese such that it is inertduring the polycondensation reaction, particularly when the polyester isto be employed in applications where color is important. Unsequesteredmanganese produces a polymer with poor color, the polymer has a broadermolecular weight distribution, which is generally undesirable, and whenmanganese is active as a catalyst in the polycondensation stage, manyundesirable by-products such as oxides, carboxyl groups, etc., arecreated.

The typical sequestering agent is a multivalent phosphorous. Thus, atthe end of the ester interchange reaction or during the polycondensationreaction, a tri- or penta-valent phosphorous compound is usually added.Typical phosphorous compounds suitable as sequestering agent for themanganese are tributyl phosphate, polyphosphoric acid,triphenylphosphite, etc. It is believed that the phosphorous forms acomplex with the manganese which is very stable and thus causes themanganese to be almost unavailable for catalytic activity during thedirect esterification reaction and polycondensation reaction. On theother hand, it is believed that phosphorous does not form a stablecomplex with the lithium, cobalt, nor antimony. Thus, each of thesecompounds would be reactive whenever conditions are achieved (such astemperature) that make them a catalyst for the production of thecopolyester.

It is noted that the phosphorous complex does not sequester 100 percentof the manganese. Thus in choosing the manganese level, it must be keptin mind that use of manganese yields bad polymer color, undesirableby-products, and broad molecular weight distribution for the polymerformed. With the present invention, it is desirable to provide a balancebetween the manganese and lithium as the ester interchange catalyst suchthat the reactivity, reaction speed, and side reactions are controlledin a manner to produce a quality product. Accordingly, it is importantthat a sufficient amount of manganese be employed that will speed up theester interchange reaction beyond that which can occur when only lithiumis being employed, but, on the other hand, employing a sufficient amountof lithium to achieve good color of polymer, to avoid side reactions,and to achieve a narrower molecular weight distribution of the polymer,which are the benefits of the lithium catalyst. Moreover, the lithiumcatalyst is also active, because it has not been sequestered, during thepolycondensation reaction and thus aids in reducing the overallpolycondensation time over a catalyst system that uses antimony alone.

After the ester interchange reaction, the antimony catalyst may beadded. It is important that cobalt not be added during the esterinterchange reaction because it has been determined that an esterinterchange catalyst system of cobalt, manganese, and lithium actuallyslows down the ester interchange reaction rate and increases the esterinterchange time over that of a manganese and lithium catalyst systemand produces a gray polymer which may be unacceptable in thoseapplications where color is important. Since cobalt cannot be addeduntil after substantial completion of the ester interchange reaction,the cobalt is added after the manganese has been stabilized. Likewise,the antimony may also be added at the time of the addition of thesequestering agent or shortly thereafter.

On the other hand, the antimony catalyst may be added with the variousraw material feedstock streams in the same manner as the esterinterchange catalyst of manganese and lithium. Antimony is not effectiveduring the ester interchange reaction because the temperature of thesereactions are lower than the reactivity temperature of antimony forproducing the copolyester. Thus, the antimony can be added any timebefore or during the polycondensation reaction.

The first monomer mixture is then subjected to a polycondensationreaction to yield a copolyester and glycol. The polycondensationreaction occurs at a temperature range of between 250° C. to 310° at avacuum pressure of approximately 0.1 to 3 mm of mercury. The reaction isreversible and, therefore, glycol is continuously removed to force thecompletion of the reaction toward the production of a copolyester.

It is theorized that lithium and antimony increase the polycondensationrate and that the optional addition of between about 10 ppm and 70 ppmof cobalt, based upon the expected yield of the polyester, to thepolycondensation reaction, further increases the polycondensation rateover that of lithium and antimony, and thereby reduces thepolycondensation time further than that achieved with lithium andantimony.

Generally, using an amount of any one of the catalysts which is outsidethe ranges of the present invention is not desirable. Using an amountless than about the minimum stated for any of the catalyst generallyyields a result which is not as substantial as that obtained with thepresent invention. Using an amount more than about the maximum statedfor any of the catalyst produces undesirable effects such as poor color,unwanted side products, high cost, etc.

EXPERIMENTAL PROCEDURE

An autoclave batch may be prepared in which a batch of roughly 1000grams of polymer, for example, may be produced at approximately 2.5 moleof DMT and 1.6 mole dimethyl isophthalate (DMI) and 8.0 mole of glycol.The autoclave may be first charged with the raw materials including DMT,DMI, ethylene glycol and the catalyst. Titanium dioxide may also addedin the initial charge as a delustrant. The catalysts, manganese,lithium, or cobalt, may be added in the form of acetates, and antimonyin the form of oxide, with the amount of catalysts added being basedupon the metals themselves. The autoclave may then be heated toapproximately 155° C. at atmospheric pressure where initiation of theester interchange begins.

During charging of the raw materials, the autoclave may be subjected toan inert gas (nitrogen at 4 standard cubic feet per hour) to aid inpreventing oxidation. Generally, the autoclave may be agitated with astirrer to assure homogenous commingling of the raw materials. At thestart of the ester interchange reaction (approximately when the reactorcontents reached 155° C.), the flow of nitrogen gas may be terminated.The autoclave temperature during ester interchange would probably risefrom approximately 155° C. to about 240° C. During the ester interchangethe alcohol should be continuously removed to force the reaction towardthe production of the monomer. When the ester interchange reaction issubstantially complete polyvalent phosphorous (for example,tributylphosphate) may be added to sequester the manganese. During theaddition and mixing of the phosphorous compound the nitrogen gas mayonce again turned on.

Polycondensation may be performed in the same autoclave. The autoclaveshould be heated to about 270° C. to about 290° C. thereby initiatingthe polycondensation reaction. The polycondensation reaction shouldproceed until substantial completion, during which the glycol formedshould be removed.

EXAMPLE

In view of the foregoing, it is suggested that a suitable copolyetsterof DMT and DMI may be obtained by reacting the following reactants:

    ______________________________________                                        DMT              2.4    mol    465.6  g                                       DMI              1.6    mol    310.4  g                                       EG               8.0    mol    497    g                                       Mn Ac.sub.2      0.18   g      52.5   ppm                                                                    (total polymer wt.)                            Co Ac.sub.2      0.06   g      18.5   ppm                                     Ti O.sub.2       2.33   g      0.30%                                          Li Ac.sub.2                    100    ppm                                     Sb.sub.2 O.sub.3 0.33   g      360    ppm                                     PPA              0.113  g      54     ppm                                     (polyphosphoric acid)                                                         ______________________________________                                    

It is suggested the DMT, EG and DMI and the manganese and lithiumcatalysts TiO₂ be fed to an autoclave for ester interchange reaction.The temperature of the autoclave should be raised above 240° C. andcontinuously refluxed until completion of the ester interchange. Thenthe PPA and cobalt and antimony catalysts are added. Vacuum let downshould be conducted followed by temperature increase of the reactedmonomers to 282° C. for polymerization. This process is run untilcompletion.

Thus, it is apparent that there has been provided, in accordance withthe invention, a catalyst system in combination with a method ofpreparing copolyester from two lower dialkyl esters of a dicarboxylicacid and glycol, using the catalyst system that fully satisfies theobjects, aims and advantages as set forth above. While the invention hasbeen described in conjunction with specific embodiments thereof, it isevident that many alternatives, modifications and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, it is intended to embrace all suchalternatives, modifications and variations that fall within the spherean scope of the invention.

That which is claimed is:
 1. In a process for making a copolyester fromat least two lower dialkyl esters of a dicarboxylic acid having theformula ##STR3## wherein R is selected from the group consisting of##STR4## and --(CH2)_(n) -- where n=2-12 and a glycol comprising thesteps of: a. reacting said lower alkyl esters of dicarboxylic acid andglycol by an ester interchange reaction to produce a monomer mixture andalcohol;b. removing said alcohol during said ester interchange reaction;c. polymerizing the monomer mixture by a polycondensation reaction toproduce said copolyester and glycol; and d. removing said glycol duringsaid polycondensation reaction;wherein the improvement comprises: addingan effective catalytic amount of manganese and lithium before or duringsaid ester interchange reaction; and adding an effective catalyticamount of antimony at or before the beginning of the polycondensationreaction.
 2. In the process of claim 1 wherein said manganese, lithium,and antimony are in the form of salts.
 3. In the process of claim 1,wherein said manganese, lithium, and antimony are in the form of organiccompounds or inorganic compounds.
 4. In the process of claim 3, whereinsaid organic compounds are selected from the class of carboxylatedmetallic salts or metal amines.
 5. In the process of claim 3, whereinsaid inorganic compounds are selected from the class of metal halidesand metal compounds of Group IV.
 6. In the process of claim 1, whereinsaid manganese, and lithium are in the form of acetates and saidantimony is in the form of oxide.
 7. In the process of claim 1, whereinone of said lower dialkyl esters is dimethyl terephthalate, and saidglycol is ethylene glycol.
 8. In the process of claim 1, wherein saidester interchange reaction occurs at a temperature range of from about150° C. to about 250° C., and at about atmospheric pressure.
 9. In theprocess of claim 8, wherein said polycondensation reaction occurs at atemperature range of from about 250° C., and at a pressure of from about0.1 to about 3.0 mm mercury vacuum.
 10. In the process of claim 1,wherein said manganese is present in a range of from about 20 ppm toabout 150 ppm, said lithium is present in a range of from about 50 ppmto about 350 ppm, and said antimony is present in a range of from about200 ppm to about 400 ppm, wherein all amounts are based on the expectedyield of said copolyester.
 11. In the process of claim 1, wherein saidmanganese is sequestered after said ester interchange reaction issubstantially completed or during said polycondensation reaction byadding a sequestering agent.
 12. In a process for making a copolyesterfrom at least two lower dialkyl esters of a dicarboxylic acid having theformula ##STR5## wherein R is a selected from the group consisting of##STR6## and --(CH2)_(n) -- where n=2-12 and a glycol comprising thesteps of:a. reacting the lower alkyl esters of dicarboxylic acid andglycol by an ester interchange reaction to produce a monomer mixture andalcohol; b. removing said alcohol during said ester interchangereaction; c. polymerizing said monomer mixture by a polycondensationreaction to produce said copolyester and glycol; and d. removing saidglycol during said polycondensation reaction;wherein the improvementcomprises: adding an effective catalytic amount of manganese and lithiumbefore or during said ester interchange reactions; and adding aneffective catalytic amount of cobalt and antimony at or before thebeginning of or during the polycondensation reaction, with the provisothat said cobalt not be added prior to substantial completion of saidester interchange reactions.
 13. In the process of claim 12, whereinsaid manganese is present in a range of from about 20 ppm to about 150ppm, said lithium is present in a range of from about 50 ppm to about350 ppm, said cobalt is present in a range of from about 10 ppm to about70 ppm, and said antimony is present in a range of from about 200 ppm toabout 400 ppm, wherein all amounts are based on the expected yield ofsaid polyester.
 14. A copolyester made from the process of claim
 1. 15.A copolyester made from the process of claim
 10. 16. A copolyester madefrom the process of claim
 11. 17. A copolyester made from the process ofclaim
 12. 18. An enhanced bicomponent fiber existing in a sheath/corerelationship comprisinga core component of polyester; and a sheathcomponent being a copolyester made from the process of claim
 1. 19. Anenhanced bicomponent fiber existing in a sheath/core relationshipcomprisinga core component of polyester; and a sheath component being acopolyester made from the process of claim
 10. 20. An enhancedbicomponent fiber existing in a sheath/core relationship comprisingacore component of polyester; and a sheath component being a copolyestermade from the process of claim
 12. 21. A fiberfill blend for making intoa nonwoven fabric for heat bonding of said fabric, said blend consistingessentially offrom 55 to 97% by weight of a crimped polyester fiber andfrom 3 to 45% by weight of a crimped binder fiber comprising acopolyester made from the process of claim
 1. 22. A fiberfill blend formaking into a nonwoven fabric for heat bonding of said fabric, saidblend comprising from 55 to 97% by weight of a crimped polyesterfiberand from 3 to 45% by weight of a bicomponent fiber existing in asheath/core relationship comprising a core component of polyester; and asheath component being a copolyester made from the process of claim 1.23. An enhanced copolyester of polyethylene terephthalate/isophthalatecomprising from about 20 ppm to about 150 ppm Mn, from about 50 ppm toabout 350 ppm Li, and from about 200 ppm to about 400 ppm antimony.